Overhead projectors (hereinafter also referred to as OHPs) are widely used in conferences and seminars to visually present charts, graphs, and the like to the audience concurrently with oral explanations. An OHP projects enlarged images onto a screen from a transparent original such as a transparency or a liquid crystal panel. To achieve this, an OHP is provided with a projection lens for projecting images onto a screen from a transparent original, and a reflection mirror for reflecting the light from a light source toward the screen. Inconveniently, however, these components are usually arranged above the body of the OHP by the use of support members, and thus they often obstruct part of the view of the images projected onto the screen.
Moreover, in conventional projection apparatuses including liquid crystal projectors, images are usually projected from a direction perpendicular to the screen onto the front surface of the screen by the projection apparatus. As a result, conventional projection apparatuses partially obstruct the projected images. On the other hand, a projector of a backward projection type that is placed behind the screen is designed to receive the light from an image from its side and reflect the light onto the back surface of the screen. As a result, projectors of this type suffer from their relatively large sizes required to allow the reflection mirror to be inclined within a sufficiently wide range of angles.
To overcome this inconvenience, various attempts have been made. For example, in the case of an OHP, as shown in FIG. 1, it is possible to prevent obstruction of the sight of observers 13 by projecting images obliquely from the OHP 11 onto the screen 3. In FIG. 1, numeral 12 represents the operator of the OHP 11. In this way, as shown in FIG. 2, in a case where the width of the screen 3 is 1.2 m and the distance from the OHP 11 to the screen 3 is 1.5 m, it is possible to observe projected images without substantial obstruction even from behind the OHP 11 by setting the projection angle .alpha. to 20.degree.. Here, the projection angle .alpha. refers to the angle formed by the line connecting the center of the projection lens of the OHP 11 to the center of the screen 3 with respect to a normal to the screen 3, as observed on the plane including the line and the normal.
Alternatively, as shown in FIG. 3, it is possible to project images obliquely upward with a projection angle .theta. by use of a similar optical system. Alternatively, as shown in FIG. 4, it is possible even to realize a low-profile projector of a backward projection type by an effective use of an optical system designed for oblique projection. In FIG. 4, numeral 1 represents a member serving as an original such as a liquid crystal panel, and numeral 7 represents a projection lens unit.
To make it possible to project images obliquely onto the surface of a screen without substantial distortion as described above, various inventions have been made. For example, Japanese Published Patent No. H4-56297 proposes using a means for movably supporting a lens and a means for movably supporting an original stand to allow the lens and the original stand to be inclined at desired angles so that images can be projected without substantial distortion. On the other hand, Japanese Laid-open Patent Application No. H5-241096 proposes arranging the surface of a screen at an angle with respect to a projection lens and in addition, to correct the resulting trapezoid distortion, arranging part of the projection lens at an angle.
However, the construction proposed in Japanese Published Patent No. H4-56297 requires a large-aperture wide-angle projection lens to obtain projection angles larger than 20.degree., and thus requires extra cost. In addition, as shown in FIG. 5, with such large projection angles, this construction exhibits markedly asymmetrical distribution of projected light with respect to the center of the image plane on the screen 3, because it has its aperture diaphragm 8 arranged perpendicularly to the optical axis of the projection lens unit 7.
Moreover, in an ordinary optical system, a change in the projection distance is coped with by adjusting focus, that is, by moving the projection lens along the optical axis, but, in an oblique projection optical system, as shown in FIG. 6, moving the projection lens unit 7 and the aperture diaphragm 8 from their positions as originally designed along the optical axis as indicated by the arrow H results in eclipse in that part of the light from the light source 6 which enters the optical system from oblique directions; specifically, the part of the light that is expected to pass through the hatched region B is eclipsed, and thus this optical system suffers from heavy loss and uneven distribution of light. In FIG. 6, numeral 5 represents a condenser lens for condensing the light from the light source 6.
On the other hand, the construction proposed in Japanese Laid-open Patent Application No. H5-241096 is difficult to adapt to large projection angles, because inclining part of the projection lens leads to a serious degradation in its optical performance.